Thermal decomposition of nickel nitrate hexahydrate, Ni(NO3)2·6H2O, in comparison to Co(NO3)2·6H2O and Ca(NO3)2·4H2O

Article abstract of DOI:10.1016/j.tca.2007.01.031

The thermal decomposition of Ni(NO₃)₂·6H₂O (1), Ca(NO₃)₂·4H₂O (2) and nitryl/nitrosyl nitrato nickelate(II), NO₂/NO[Ni(NO₃)₃] (3), was investigated by therm

Full text of DOI:10.1016/j.tca.2007.01.031

Thermochimica Acta 456 (2007) 64–68
Thermal decomposition of nickel nitrate hexahydrate, Ni(NO3)2·6H2O,
in comparison to Co(NO3)2·6H2O and Ca(NO3)2·4H2O
Wolfgang Brockner∗, Claus Ehrhardt1, Mimoza Gjikaj2
Institute of Inorganic and Analytical Chemistry, Clausthal University of Technology, Paul-Ernst-Strasse 4, D-38678 Clausthal-Zellerfeld, Germany
Received 20 December 2006; received in revised form 9 January 2007; accepted 24 January 2007
Available online 3 February 2007
Abstract
The thermal decomposition of Ni(NO3)2·6H2O (1), Ca(NO3)2·4H2O (2) and nitryl/nitrosyl nitrato nickelate(II), NO2/NO[Ni(NO3)3] (3), was
investigatedbythermogravimetricmeasurementswithquasi-isothermalconditionsandcomparedtoCo(NO3)2·6H2O. Therespectivedecomposition
processes of 1 and 2 differ from each other showing that at one hand anhydrous Ca(NO3)2 was obtained whereas anhydrous nickel dinitrate has
not be formed due to redox and condensation reactions. Instead basic nickel compounds have been formed. In reducing atmosphere nickel metal
can be obtained. Anhydrous Ni(NO3)2 results by the thermal degradation of nitryl/nitrosyl nitrato nickelate. FT-Raman spectra have been of help
in the judgement of the decomposition processes.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ni(NO3)2·6H2O; Ca(NO3)2·4H2O; NO2/NO[Ni(NO3)3]; Thermochemistry
1. Introduction
utive reactions are outlined in a series of papers of A. Malecki
and co-workers [12–14,17] including the formation of HNO3,
NO, NO2 and O2 as well as the formation of hydroxide nitrate
hydrates, hydroxide nitrates, basic nitrates and oxides depen-
dent on experimental conditions inclusive formation of hydrate
melts. In some cases anhydrous metal nitrates can be obtained as
intermediates(e.g. barium, calcium, cadmiumandcobaltnitrate)
[14,19,20] whereas in other cases basic oxo- or hydroxo-nitrates
are formed (e.g. copper, nickel and manganese compounds)
[2,6,8,13,15,17,21–23]. In a continuation of our investigation
of the thermal decomposition of cobalt(II)nitrate hexahydrate
[20] we investigated the title nitrates, especially in relation to
their decomposition reactions and chemism.
Metal nitrates (hydrates) are most popular precursor compo-
nents for the preparation of oxidic and metallic materials with
wide spread properties as e.g. high surface catalysts, ceramics,
semiconductors, gas sensors and high temperature superconduc-
tors [1–9]. The widely used procedures thereby are the thermal
degradation/decomposition of metal nitrates as well as chemi-
cal vapor deposition (CVD) methods. The actual decomposition
processes often depend very much upon the experimental condi-
tions employed as for example heating rate, amount and packing
of starting material, kind and pressure of the gas atmosphere, gas
flow rate, melting/formation of a hydrate melt, and others more.
Dependent on the applied preparation procedures thereby,
different decomposition reactions and mechanisms result and
are disputed for the thermal degradation respective decom-
position [3,10–18]. Different routes of thermal decomposition
especially of transition metal nitrate hydrates in form of consec-
2. Experimental
2.1. Substances and methods
Commercial nickel nitrate hexahydrate (Fluka, puriss. p.a.)
and calcium nitrate tetrahydrate (Fluka, puriss. p.a.) (Fluka,
purum p.a.) were used as received. NO2/NO[Ni(NO3)3] was
prepared out of NiCl2 and N2O5 according to ref. [24].
The thermogravimetric (TG) measurements have been per-
formed with a thermobalance (Sartorius, type 4201) in which
the sample compartment is separated and kept magnetically in
∗
Corresponding author. Tel.: +49 5323 722656; fax: +49 5323 722995.
E-mail addresses: wolfgang.brockner@tu-clausthal.de (W. Brockner),
claus.ehrhardt@tu-clausthal.de (C. Ehrhardt), mimoza.gjikaj@tu-clausthal.de
(M. Gjikaj).
1
Tel.: +49 5323 722484; fax: +49 5323 722995.
Tel.: +49 5323 722887; fax: +49 5323 722995.
2
0040-6031/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2007.01.031

W. Brockner et al. / Thermochimica Acta 456 (2007) 64–68
65
Fig. 2. Thermal decomposition (TG) of nickel(II)nitrate hexahydrate (full line)
in comparison to cobalt nitrate hexahydrate (dotted/broken line) (c.f. ref. [20])
in N2 atmosphere up to 400 ◦C with indicated degradation steps/stages. Sample
Fig. 1. Thermal decomposition (TG) of calcium nitrate tetrahydrate in H2/N2
(10% H2) atmosphere with indicated degradation steps/stages. Loading: 150 mg;
loading: 150 mg; heating rate between the quasi-isothermal steps: 0.6 K min−1
.
heating rate between the quasi-isothermal steps: 0.6 K min−1
.
The different behavior of both nitrates is seen in the third step respective stage.
CaO. This decomposition course is typical for nitrates with hard-
oxidable respective unoxidable cations as alkali, alkaline earth,
Cd2+, Pb2+, Zn2+ and others. The used atmosphere, here H2/N2
(10% H2), has no influence on it. The corresponding experi-
mental details and thermo-analytical characters including the
chemical reactions are summerized in Table 1 and indicated
in Fig. 1, respectively. Nevertheless, high sample loading and
dense packing may hinder the leaving of water and cause the
formation of side-products, too. Such effects we discussed for
Co (NO3)2·6H2O in ref. [20]. The insignificant “step” in the
mass loss curve between Ca(NO3)2 and CaO (Fig. 1) could be
caused by short electronic net pertubation.
suspension (Magnetic Suspension Balance). By a computer-
based driving, the heating rate is controlled by relative mass
loss in response to the sample decomposition allowing so-called
quasi-isothermal conditions [25]. This method is in effect com-
parable to the “Dynamic Rate Controlled Method” [26]. With
the mentioned thermobalance also thermal decomposition reac-
tions evolving corrosive gases as e.g. HNO3, NOX, Cl2, etc., can
be pursued as well as it is possible to work in different atmo-
spheres. The method’s efficiency is outlined for some effects of
the thermal degradation of Co(NO3)2·6H2O [20].
TheTGmeasurementsweredoneinaflowinggasatmosphere
of N2 respective H2/N2 (10% H2). The choice of the applied
atmosphere was made in order to evaluate a possible influence
on partial decomposition steps.
3.2. Thermal decomposition of Ni(NO3)2·6H2O in
comparison to Co(NO3)2·6H2O [20]
3. Results and discussion
In general it is agreed that the thermal degrada-
tion/decomposition of nickel(II)nitrate hexahydrate proceeds
stepwise and the distinct tetra- and dihydrates are formed if
melting respective formation of a hydrate melt (m.p. 56.7 ◦C)
is prevented [1–3,6,11,13,17,27–28] (c.f. Fig. 2). Besides this,
recently a proof of the existence of the controversial intermediate
nickel nitrate salt Ni(NO3)2·5.5H2O is claimed [29].
3.1. Thermal decomposition of calcium nitrate
tetrahydrate, Ca(NO3)2·4H2O
The slow thermal decomposition of Ca(NO3)2·4H2O (2),
shown in Fig. 1, reveals the stepwise formation of distinct
hydrates, anhydrous calcium nitrate and finally calcium oxide,
Table 1
Thermal decomposition reactions of Ca(NO3)2·4H2O and their characteristics (c.f. Fig. 1)
Reaction/step
T (K)a
ꢀm (%)b
Experimental
(Calculated)
1. Ca(NO3)2·4H2O = Ca(NO3)2·3H2O + H2O
2. Ca(NO3)2·3H2O = Ca(NO3)2·2.5H2O + 0.5H2O
3. Ca(NO3)2·2.5H2O = Ca(NO3)2 + 2.5H2O
318
∼340
427
7.5
11.2
30.3
76.1
(7.63)
(11.47)
(30.50)
(76.25)
c
4. Ca(NO3)2 = CaO + N2O5
805
a
Decomposition temperature.
In relation to the starting compound.
Respective 2NO2 + 0.5O2.
b
c

66
W. Brockner et al. / Thermochimica Acta 456 (2007) 64–68
Fig. 4. Thermal decomposition (TG) of nitryl/nitrosyl nitrato nickelate in N2
atmosphere up to 500 ◦C with indicated degradation steps/stages. Sample load-
Fig. 3. Thermal decomposition (TG) of nickel(II)nitrate hexahydrate in inert/N2
atmosphere (full line) and in reducing/H2/N2 (10% H2) atmosphere (dot-
ted/broken line) with indicated degradation steps/stages. Heating rate between
ing: 120 mg; heating rate between the quasi-isothermal steps: 0.6 K min−1
.
the quasi-isothermal steps: 0.6 K min−1
.
Otherwise, the nitrate ion has a considerable oxidative power
and it is possible that Ni2+ can be oxidized to Ni3+, what is
indicated in refs. [2,13,17] and proved for [Ni(NH3)6](NO3)2
where the thermal decomposition step of [Ni(NH3)2](NO3)2 is
connected with a redox process [30]. It is possible to formu-
late the ongoing Ni(NO3)2·2H2O decomposition reasonably via
an oxidation step (Table 2, Reaction 3a) followed by a partial
condensation (Reaction 3b) with the release of 0.25 equivalents
H2O. The used atmosphere has no influence on the decompo-
sition course, except for the final step, the formation of nickel
oxides respective elemental nickel (Fig. 3).
Noteworthy, the third degradation step of the nickel com-
pound differs from this of the cobalt homologue [20] and it
results in basic species, and not in anhydrous Ni(NO3)2!! The
statements for the composition of such basic products dif-
fer [6,7,21,27]. A homogenous single phase Ni3(NO3)2(OH)4
[=Ni(NO3)2·2Ni(OH)2] could be characterised recently [7]. The
formation of the mentioned basic nickel salt corresponds (at
least formal) to an hydrolysis of nitrate and the release of HNO3
according to
Concerning the nickel oxide(s) decomposition we have only
checked the existence of NiO (by X-ray diffraction) at the
end of the thermal decomposition. It was not tried to obtain
and to characterize a Ni2O3 sample. In the high-temperature
oxide decomposition (Figs. 2–4) the mass loss course proceeds
3Ni(NO3)2·2H2O = Ni3(NO3)2(OH)4 + 4HNO3 + 2H2O
corresponding to the calculated mass loss of 57.80%. Such a
mass loss could not be detected in the experimental decomposi-
tion course (c.f. Figs. 2 and 3).
Table 2
Thermal decomposition reactions of Ni(NO3)2·6H2O and their characteristics (c.f. Figs. 2 and 3)
Reaction/step
T (K)a
ꢀm (%)b
Experimental
(Calculated)
Water separation
1. Ni(NO3)2·6H2O = Ni(NO3)2·4H2O + 2H2O
316
353
12.3
24.8
(12.38)
(24.77)
2. Ni(NO3)2·4H2O = Ni(NO3)2·2H2O + 2H2O
Partial decomposition steps (oxidation and partial condensation)
3a. Ni(NO3)2·2H2O = Ni(NO3)(OH)2·H2O + NO2
418
∼463
40.6
42.0
(40.59)
(42.14)
3b. Ni(NO3)(OH)2·H2O = Ni(NO3)(OH)1.5O0.25·H2O + 0.25H2O
Decomposition
4. Ni(NO3)(OH)1.5O0.25·H2O = 0.5Ni2O3 + HNO3 + 1.25H2O
523
71.8
(71.55)
Oxide decomposition to “NiO”
5. 3Ni2O3 = 2Ni3O4 + 0.5O2
6. Ni3O4 = 3NiO + 0.5O2
∼523c
∼573c
72.2
74.0
(72.47)
(74.31)
Reduction with H2/N2 (10% H2) to Ni, from 4
3NiO + 3/2H2 = 3Ni + 3/2H2O (Fig. 3)
∼535
79.8
(79.81)
a
Decomposition temperature.
In relation to the starting compound.
b
c
Beginning of reaction.

W. Brockner et al. / Thermochimica Acta 456 (2007) 64–68
67
Table 3
Thermal decomposition reactions of nitryl/nitrosyl nitrato nickelates and their characteristics in N2 atmosphere (c.f. Fig. 4)
Reaction/step
T (K)a
ꢀm (%)b
Experimental
(Calculated)
1. NO2[Ni(NO3)3] = NO[Ni(NO3)3] + 0.5O2
2. NO[Ni(NO3)3] = Ni(NO3)2O0.5 + NO + 0.5N2O5
3. Ni(NO3)2O0.5 = Ni(NO3)2 + 0.25O2
4. Ni(NO3)2 = 0.5Ni2O3 + NO2 + 0.5N2O5
5. Ni2O3 decomposition via Ni3O4 in NiO
293
413
5.0
32.2
38.0
72.0
74.2
(5.50)
(34.40)
(37.15)
(71.55)
(74.31)
∼423
∼540
above 593
The thermal decomposition of Ni(NO3)2 in H2/N2 atmosphere beginning at 255 ◦C yields elemental nickel
a
Decomposition temperature.
In relation to the starting compound.
b
dawdlingly ending up with NiO. The labels Ni2O3 and Ni3O4
correspond to the respective mass loss fitting with the experi-
mental ones.
Such an approach will better harmonize with the thermal
decomposition of manganese(II)nitrate hydrates for example
[22,23] whereat MnO2 is obviously formed. Unfortunately,
the presented thermal decomposition measurements cannot
contribute to a solution of the decomposition mechanism by
methodical reasons.
formed. In the relevant composition range no distinct phases
could be characterised by X-ray diffraction.
Therefore, the reaction mechanism of the thermal decom-
position of Ni(NO3)2·2H2O to basic intermediates either by
hydrolysisofnitrateionorbyredoxreactionsisfinallyunproved.
For further decomposition at higher temperatures the used atmo-
sphere is important, whereas in N2 nickel oxides and in H2/N2
(10%H2) elemental nickel are formed. Anhydrous nickel nitrate,
Ni(NO3)2, can be obtained in substance by thermal decomposi-
tion of nitryl/nitrosyl nitrato nickelate (NO2/NO)[Ni(NO3)3].
At least some contradictory (and non-reproducible) results
and findings of the thermal Ni(NO3)2·6H2O decomposition
might be caused by hasty temperature rising respective for-
mation of a hydrate melt changing the system drastically by
incoupled side reactions and evaporation of volatile species.
3.3. Thermal decomposition of nitryl/nitrosyl nitrato
nickelate (NO2/NO)[Ni(NO3)3]
Since anhydrous Ni(NO3)2 can not be obtained by ther-
mal dehydration of the title compound under usual conditions
as outlined, it can be prepared by thermal decomposition of
nitryl/nitrosyl nitrato nickelates, NO2/NO [Ni(NO3)3], under
milder conditions (Fig. 4, Table 3). The thermal degradation of
the nitryl/nitrosyl nitrato nickelate is included here in order to
show that anhydrous Ni(NO3)2 can be obtained in substance and
thatithasaconsiderablethermalstability. Thissoobtainedanhy-
drous nickel dinitrate is characterised by its Raman spectrum
(see footnote 3).3 The Raman frequencies of the nickel(II)nitrate
are very similar to those of anhydrous cobalt(II)nitrate [20] and
they may also assigned in analogy based on the vibrations of the
nitrate unit.
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4. Conclusion
The thermal dehydration/degradation of Ca(NO3)2·4H2O
under quasi-isothermal conditions proceeds stepwise via dis-
tinct hydrates, anhydrous Ca(NO3)2 and finally CaO. This
decomposition course is typical for nitrate hydrates with
hard-oxidable mono- and divalent cations (but not for e.g.
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Raman frequencies of anhydrous nickel(II)nitrate, Ni(NO3)2: –NO3 stretch-
ings 1451w-m, 1383w, 1354w, 1092sh, 1083vs, 1056w; –NO3 bendings 758w,
748w; lattice vibrations 208m, 185vw, 160m, 147m, 122sh, 86vw, with
s = strong, m = medium, w = weak, v = very, sh = shoulder.

68
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